The present invention is directed to a micro-electromechanical drop ejector that can be used for direct marking. The ink drop is ejected by the piston action of an electrostatically or magnetostatically deformable membrane. The new feature of the invention is that it is easily fabricated in a standard polysilicon surface micromachining process, and can thus be batch fabricated at low cost using existing external foundry capabilities. In addition, the surface micromachining process has proven to be compatible with integrated microelectronics, allowing for the monolithic integration of the actuator with addressing electronics. In contrast to the magnetically actuated drop ejector described in U.S. patent application Ser. No. 08/869,946, entitled xe2x80x9cA Magnetically Actuated Ink Jet Printing Devicexe2x80x9d, filed on Jun. 5, 1997, now U.S. Pat. No. 6,234,608 and assigned to the same assignee as the present invention, the electrostatically actuated version of the present invention does not require external magnets for actuation of the diaphragm, and does not have the ohmic-losses that arise from the flow of current through the coil windings.
Current Thermal Ink Jet (TIJ) direct marking technologies are limited in terms of ink latitude, being limited to aqueous based inks, and productivity, by the high-power requirements associated with the water-vapor phase change in both the drop ejection and drying processes. The limitation to aqueous based inks leads to limitations in image quality and image quality effects due to heating of the drop ejector. The requirements for high-power in the drop ejection process limits the number of drop ejectors that can be fired simultaneously in a Full-Width Array (FWA) geometry, that is required for high productivity printing. The requirement for high-power drying to evaporate the water in aqueous based inks also leads to limitations in high productivity printers. It is very likely that the next breakthrough in the area of direct marking will be in the area of inks, such as non-aqueous and liquid-solid phase change inks, and a drop ejector with sufficient ink latitude would be the enabler for the use of such inks.
U.S. Pat. Nos. 5,668,579, 5,644,341, 5,563,634, 5,534,900, 5,513,431, 5,821,951, 4,520,375, 5,828,394, 5,754,205 are drawn to microelectromechanical fluid ejecting devices. In the majority of these patents, the ejector is fabricated using bulk micromachining technology. This processing technology is less compatible with integrated electronics, and thus is not cost effective for implementing large arrays of drop ejectors which require integrated addressing electronics and also has space limitations due to sloped walls. The surface micromachining process of the present invention described above is compatible with integrated electronics. This is a very important enabler for high-productivity full-width array applications. An additional feature described above is the xe2x80x9cnipplexe2x80x9d or landing foot of the present invention. This feature is important for keeping the membrane from contacting the counter-electrode in device operation. The Seiko-Epson device described in the above patents does not have this feature and they must include an insulating layer between the membrane and counter-electrode in order to avoid electric contacts. This insulating layer has a tendency to collect injected charge, which leads to unreproducable device characteristics unless the device is run in a special manner, as described in U.S. Pat. No. 5,644,341. An additional feature of the present invention described above is using a charge drive mode in order to enable gray level printing using multiple drop sizes. The charge drive mode allows the membrane to be deformed to a user selected amplitude, rather than being pulled all of the way down by the familiar xe2x80x9cpull-inxe2x80x9d instability of the voltage drive mode. Finally, the device of the present invention can be implemented as a monolothic ink jet device, not requiring the high-cost wafer bonding techniques used in the Seiko-Epson patents. The nozzle plate and pressure chamber can be formed directly on the surface of the device layer using either an additional polysilicon nozzle plate layer, or a thick polyimide layer as described in U.S. patent application Ser. No. 08/905,759 entitled xe2x80x9cMonolithic Ink Jet Printheadxe2x80x9d to Chen et al., filed Aug. 4, 1997, now U.S. Pat. No. 6,022,482 and assigned to the same assignee as the present invention, or U.S. Pat. No. 5,738,799, entitled, xe2x80x9cMethod and Materials for Fabricating an Ink-Jet Printheadxe2x80x9d, also assigned to the same assignee as the present invention or as described in a publication entitled xe2x80x9cA Monolithic Polyimide Nozzle Array for lnkjet Printingxe2x80x9d by Chen et al., published in Solid State Sensor and Actuators Workshop, Hilton Head Island, S. C., Jun. 8-11, 1998. This is an important enabler for bringing down manufacturing cost.
U.S. Pat. Nos. 5,867,302, 5,895,866, 5,550,990 and 5,882,532 describe other micromechanical devices and methods for making them.
All of the references cited in this specification are hereby incorporated by reference.
The present invention increases ink latitude by eliminating the need for the liquid-vapor phase change in thermal ink jets, and decreases power consumption by three orders of magnitude by using mechanical rather than thermal actuation, and non-aqueous based inks.